Review the characteristics of the various network topologies, including their strengths and weaknesses.

Review the characteristics of 802.11 standards, including the information provided in all tables in this chapter.

Identify the components involved in wireless communications.

Review the factors that cause wireless interference.

Review the Notes, Tips, and Exam Alerts in this chapter. Be sure that you understand the information in the Exam Alerts. If you don’t understand the topic mentioned in an Exam Alert, reread that information in the chapter, and then reread the Exam Alert.

Introduction

One of the bigger changes in the networking world since the release of the previous Network+ is in wireless networking. Networks of all shapes and sizes incorporate wireless segments into their networks. Home wireless networking has also grown significantly in the last few years.

Wireless networking enables users to connect to a network using radio waves instead of wires. Network users within range of a wireless access point (AP) can move around an office freely without needing to plug into a wired infrastructure. The benefits of wireless networking clearly have led to its growth.

Today, wireless local area networks (WLANs) provide a flexible and secure data communications system that augments an Ethernet LAN or, in some cases, replaces it. Wireless transmissions send and receive data using radio frequency (RF) signals, freeing us from wired solutions.

In a common wireless implementation, a wireless transceiver (transmitter/receiver), known as an access point, connects to the wired network from a fixed location using standard cabling. The wireless access point receives and then transmits data between the wireless LAN and the wired network infrastructure.

Client systems communicate with a wireless access point using wireless LAN adapters. Such adapters are built into or can be added to laptops, PDAs, or desktop computers. Wireless LAN adapters provide the communication point between the client system and the airwaves via an antenna.

This chapter explores the many facets of wireless networking, starting with some of the concepts and technologies that make wireless networking possible.

Note: Ad hoc and infrastructure

This chapter discusses ad hoc and infrastructure wireless network topologies. If you need a refresher on these topologies, they are discussed in Chapter 1, “Introduction to Networking.”

Wireless Access Points

As discussed in Chapter 3, “Networking Components and Devices,” a wireless access point (AP) is both a transmitter and receiver (transceiver) device used for wireless LAN (WLAN) radio signals. An AP typically is a separate network device with a built-in antenna, transmitter, and adapter. APs use the wireless infrastructure network mode to provide a connection point between WLANs and a wired Ethernet LAN. APs also typically have several ports, giving you a way to expand the network to support additional clients.

Depending on the size of the network, one or more APs might be required. Additional APs are used to allow access to more wireless clients and to expand the range of the wireless network. Each AP is limited by a transmission range—the distance a client can be from an AP and still get a usable signal. The actual distance depends on the wireless standard being used and the obstructions and environmental conditions between the client and the AP. Factors affecting wireless transmission ranges are covered later in this chapter.

Note: Wireless access points

An AP can operate as a bridge, connecting a standard wired network to wireless devices, or as a router, passing data transmissions from one access point to another.

Tip: AP range

If you are using a wireless device that loses its connection, you might be too far from the AP.

As mentioned in Chapter 1, an AP can be used in an infrastructure wireless network design. Used in the infrastructure mode, the AP receives transmissions from wireless devices within a specific range and transmits those signals to the network beyond. This network might be a private Ethernet network or the Internet. In infrastructure wireless networking, there might be multiple access points to cover a large area or only a single access point for a small area, such as a single home or small building.

Working with APs

When working with wireless APs, you need to understand many terms and acronyms. This section defines some of the more common wireless acronyms you will see both on the exam and in wireless networking documentation.

Exam Alert

Several of the acronyms provided in the following list are sure to be on the Network+ exam. Be sure you can identify the function of each before taking the exam.

Service Set Identifier (SSID)—A network name needed to connect to a wireless AP. It is like a workgroup name used with Windows networking. 802.11 wireless networks use the SSID to identify all systems belonging to the same network. Client stations must be configured with the SSID to be authenticated to the AP. The AP might broadcast the SSID, allowing all wireless clients in the area to see the AP’s SSID. For security reasons, APs can be configured not to broadcast the SSID or to cloak it. This means that an administrator needs to give client systems the SSID instead of allowing it to be discovered automatically.

TIP

One element of wireless security involves configuring the AP not to broadcast the SSID. This configuration is made on the AP.

Basic Service Set (BSS)—Refers to a wireless network that uses a single AP and one or more wireless clients connecting to the AP. Many home offices are an example of a BSS design. The BSS is an example of the infrastructure wireless topology. Wireless topologies and other network topologies are discussed in Chapter 1.

Extended Service Set (ESS)—Refers to two or more connected BSSs that use multiple APs. The ESS is used to create WLANs or larger wireless networks and is a collection of APs and clients. Connecting BSS systems allows clients to roam between areas and maintain the wireless connection without having to reconfigure between BSSs.

Extended Service Set Identifier (ESSID)—Although the terms ESSID and SSID are used interchangeably, there is a difference between the two. SSID is the name used with BSS networks. ESSID is the network name used with an ESS wireless network design. With an ESS, not all APs necessarily use the same name.

Basic Service Set Identifier (BSSID)—The MAC address of the BSS AP. The BSSID is not to be confused with the SSID, which is the name of the wireless network.

Basic Service Area (BSA)—When troubleshooting or designing wireless networks, the BSA is an important consideration. The BSA refers to the AP’s coverage area. The BSA for an AP depends on many factors, including the strength of the AP antenna, interference in the area, and whether an omnidirectional or directional antenna is being used.

Wireless Antennas

A wireless antenna is an integral part of overall wireless communication. Antennas come in many different shapes and sizes, with each one designed for a specific purpose. Selecting the right antenna for a particular network implementation is a critical consideration and one that could ultimately decide how successful a wireless network will be. In addition, using the right antenna can save you money on networking costs, because you need fewer antennas and access points.

Many small home network adapters and access points come with a nonupgradable antenna, but higher-grade wireless devices require you to choose an antenna. Determining which antenna to select takes careful planning and requires an understanding of what range and speed you need for a network. The antenna is designed to help wireless networks do the following:

Work around obstacles

Minimize the effects of interference

Increase signal strength

Focus the transmission, which can increase signal speed

The following sections explore some of the characteristics of wireless antennas.

Antenna Ratings

When a wireless signal is low and is being affected by heavy interference, it might be possible to upgrade the antenna to create a more solid wireless connection. To determine an antenna’s strength, we refer to its gain value. But how do we determine the gain value?

Suppose that a huge wireless tower is emanating circular waves in all directions. If we could see these waves, we would see them forming a sphere around the tower. The signals around the antenna flow equally in all directions, including up and down. An antenna that does this has a 0dBi gain value and is called an isotropic antenna. The isotropic antenna rating provides a base point for measuring actual antenna strength.

Note: dB

The dB in dBi stands for decibels, and the i stands for the hypothetical isotropic antenna.

An antenna’s gain value represents the difference between the 0dBi isotropic and the antenna’s power. For example, a wireless antenna advertised as 15dBi is 15 times stronger than the hypothetical isotropic antenna. The higher the decibel figure, the higher the gain.

When looking at wireless antennas, remember that a higher gain value means stronger send and receive signals. In terms of performance, the rule of thumb is that every 3dB of gain added doubles an antenna’s effective power output.

Antenna Coverage

When selecting an antenna for a particular wireless implementation, it is necessary to determine the type of coverage the antenna uses. In a typical configuration, a wireless antenna can be either omnidirectional or directional. Which one you choose depends on the wireless environment.

An omnidirectional antenna is designed to provide a 360-degree dispersed wave pattern. This type of antenna is used when coverage in all directions from the antenna is required. Omnidirectional antennas are advantageous when a broad-based signal is required. For example, if you provide an even signal in all directions, clients can access the antenna and its associated access point from various locations. Because of the dispersed nature of omnidirectional antennas, the signal is weaker overall and therefore accommodates shorter signal distances. Omnidirectional antennas are great in an environment that has a clear line of sight between the senders and receivers. The power is evenly spread to all points, making omnidirectional antennas well suited for home and small office applications.

Note: Directional differences

Omnidirectional antennas provide wide coverage but weaker signal strength in any one direction than a directional antenna.

Directional antennas are designed to focus the signal in a particular direction. This focused signal allows for greater distances and a stronger signal between two points. The greater distances enabled by directional antennas give you a viable alternative for connecting locations, such as two offices, in a point-to-point configuration.

Directional antennas are also used when you need to tunnel or thread a signal through a series of obstacles. This concentrates the signal power in a specific direction and allows you to use less power for a greater distance than an omnidirectional antenna. Table 7.1 compares omnidirectional and directional wireless antennas.

Omnidirectional antennas are limited to their circular pattern range. Directional antennas can be adjusted to define a specific pattern, wider or more focused.

Note: Polarization

In the wireless world, polarization refers to the direction in which the antenna radiates wavelengths. This direction can either be vertical, horizontal, or circular. Today, vertical antennas are perhaps the most common. As far as the configuration is concerned, the sending and receiving antennas should be set to the same polarization.